1. Field of the Invention
The present invention relates to a multiplex communications system in which a master node and at least two slave nodes are interconnected by way of a common multiplex communications line; i.e., a bus line. More particularly, the present invention relates to a multiplex communications system in which a master node controls all slave nodes.
The present application is based on Japanese Patent Application No. Hei. 10-141625, which is incorporated herein by reference.
2. Description of the Related Art
In the related system of this type, data can be exchanged only between a master node and a slave node. When the input level of one of the slave nodes is changed, event transmission is carried out to automatically transmit to the master node information about the change. When data are transferred to the slave node from the master node in response to event transmission, the master node transfers data by designation of the slave node, through use of a frame including the identification of the node that requires the data. In contrast, the slave node transfer data to the master node through polling. The master node periodically selects each of the slave nodes in turn. When being designated by the master node, the slave node performs data transfer operations through response transmission.
In principle, communication between the master node and the slave nodes is carried out according to one of the above two methods. Input information transferred from a slave node through polling and event transmission is collectively stored in the master node. Further, information about the output status of the slave node transferred therefrom by means of polling is also stored in the master node. When an imbalance arises between the input information and the output information, data including the input information are transferred to the slave node holding the output information corresponding to the input information to thereby cause the output information to match the input information, thus driving load according to the input information. Such a master-slave system has the merits of reducing the burden of controlling imposed on the slave node and achieving cost reduction.
In such multiplex communication, if the master node experiences a malfunction and becomes uncontrollable, all the slave nodes become inoperative.
In view of the foregoing problems, an object of the present invention is to provide a multiplex communications system which performs a backup operation corresponding to a malfunction while taking full use of the merit of multiplex communication.
According to the first aspect of the present invention, there is provided a multiplex communications system which comprises a plurality of master-slave sub-systems, each of the master-slave sub-system including one master node connected to a multiplex communications line and at least one slave node connected to the multiplex communications line, wherein the master node of each master-slave sub-system periodically transmits a signal to the slave node of the sub-system at a first given cycle, the slave node transmits a signal in response to periodic transmission of the signal from the master node, thereby establishing communication between the master node and the slave node through the multiplex communications line and exchanging data between the slave nodes, the master node of one master-slave sub-system monitors the master node of another master-slave sub-system, and is detectable that periodic transmission of a signal at the first given cycle is incorrectly carried out, wherein when the master node of the one master-slave sub-system detects that the periodic transmission of the signal at the first given cycle is incorrectly carried out, the master node, which correctly performs periodic transmission of a signal, periodically transmits a signal to both the slave nodes of both the sub-system to which the mater node belongs and another sub-system at a second given cycle which is slightly longer than the first given cycle, and specific two slave nodes of the master-slave sub-system are detectable that periodic transmission of a signal at the second given cycle is incorrectly carried out, wherein periodic transmission of a signal at a third given cycle, which is slightly longer than the second given cycle, is performed between the specific two slave nodes in response to detection in that the periodic transmission of the signal at the second given cycle is incorrectly carried out. Accordingly, even when the master node falls into malfunction, another master node can control the slave nodes as an alternative to the master node that has fallen into malfunction. Further, even if all the master nodes of the master-slave sub-systems fall into malfunction, communication between two specific slave nodes is ensured.
According to the second aspect of the present invention, preferably, the above-described multiplex communications further comprises a backup signal line disposed between the salve node which is connected to control means and serves as an input slave node and the slave node which is connected to a load to be controlled by the control means and serves as an output slave node, wherein the output slave node comprises abnormality detection means which detects an abnormality when the output slave node does not receive a transmission signal for a given time period slightly longer than the third given cycle, and backup means which enables the control means to drive a corresponding load through the backup signal line in response to detection of an abnormality by the abnormality detection means. In this configuration, when the abnormality detection means detects an abnormality from that the output slave node does not receive a transmission signal for a given time period slightly longer than the third given cycle, the backup means enables the control means to control a corresponding load by way of the backup signal line laid between the input slave node and the output slave node. Consequently, even if communication between the slave node connected to the control means and the slave node connected to a load cannot be established by way of the multiplex communications line, minimum functions can be automatically ensured.